Method and apparatus for implementing measurement based dynamic frequency hopping in wireless communication systems

ABSTRACT

Proposed is a method for reducing interference in a frequency hopping wireless communications system. In one embodiment of the present invention, a base station and a terminal station each using an orthogonal frequency division multiplexing (OFDM) technique to simultaneously measure an interference level for each system frequency and to enable high speed frequency hop pattern changes which can follow changes in desired and interfering signal levels due to changes in co-channel interference or shadow fading. The terminal station interference level measurement values are then transmitted to the base station. Next, the base station identifies each frequency hop pattern currently in use by each terminal station communicating with that base station. The base station then uses both the base station interference level measurements and the terminal station interference level measurements to identify each frequency hop pattern in which at least one of the current system frequencies should be replaced with a system frequency having a lower interference level. Next, the base station replaces no more than a predetermined number of the current system frequencies within the identified frequency hop pattern(s). The above steps are executed at each base station within the system while ensuring that nearby interfering base stations do not replace frequencies at the same time.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.09/312,279, filed May 17, 1999, and claims the benefit of U.S.Provisional Application Ser. No. 60/114,080, filed Dec. 28, 1998,entitled “Method and Apparatus for Implementing Measurement BasedDynamic Frequency Hopping In Wireless Communication Systems,” each ofwhich are incorporated herein by reference in their entireties.

BACKGROUND

[0002] The present invention is directed to frequency hopping wirelesscommunications systems. More specifically, the present invention reducesinterference levels and increases capacity in frequency hopping wirelesscommunications systems by dynamically replacing system frequencies inuse within selected frequency hop patterns with system frequencieshaving lower interference levels and by precluding nearby interferingsystem components (for example, base stations) from simultaneouslymaking frequency replacements using the same available systemfrequencies.

[0003] The demand for wireless communications services continues to growat an astonishing rate. For example, each day a greater percentage ofthe public elects untethered access to a telephone system using cellulartelephones. Unlike traditional telephones with attached cords whichlimit the user's movement, cellular telephones allow users to maketelephone calls while in transit between locations. In addition towireless voice communication services, the public is discoveringnumerous instances where wireless data communication simplifies theirlives. For example, an employee who has traveled on business away from alocal area network (LAN) in the home office may use a laptop computerhaving a radio transceiver to establish a wireless connection to the LANfrom within a hotel room. Once the wireless connection is established,the employee may check electronic mail or access other files on the LANin the same manner these tasks might be performed from within the homeoffice using a desktop computer wired to the LAN.

[0004] Unfortunately, the number of frequencies available to support thepublic's growing appetite for wireless communications services islimited. Thus, service providers must make the most efficient use ofthese frequencies to meet the growing demand. One method for increasingthe efficiency of a wireless communication system entails avoiding theuse of frequencies with high interference levels which might otherwiserequire that data be retransmitted thereby consuming additional systemresources. Some of the current wireless communication systems implementsome type of frequency hopping technique to reduce the system-wideimpact of frequencies which are experiencing high interference levels.As explained below, however, the current methods for implementingfrequency hopping techniques leave room for improvement.

[0005] Understanding the current methods for implementing frequencyhopping techniques requires a basic understanding of how typicalwireless communication systems transmit data and the major sources ofinterference within theses systems. In a typical wireless communicationsystem, a transmitter modulates a carrier frequency with voice or datainformation and transmits the modulated carrier frequency through theair to a receiver. The receiver then demodulates the carrier frequencyto obtain the included voice or data information. In some wirelesssystems, the receiver sends the transmitter a message which indicateswhether the transmitted data was successfully received. Co-channelinterference, multipath fading, and shadow fading are among the types ofinterference which may prevent the receiver from successfully receivingtransmitted data.

[0006] Co-channel interference may result when two transmitters withinrange of each other attempt to transmit data to their respectivereceivers using the same carrier frequency at the same time. The greaterthe level of co-channel interference, the greater the chance thetransmitted data will become too distorted for the receiver to process.System resources required to retransmit this data are unavailable totransmit newly arriving data. As a result, the flow of data through thesystem is slowed. As the number of users in a wireless system using thelimited available number of frequencies continues to increase, thepossibility that two or more transmitters may be located within range ofeach other and transmit data using the same frequency at the same timealso increases. Co-channel interference is particularly relevant to thedesign and deployment of cellular wireless systems.

[0007] Multipath fading occurs when a transmitted signal is reflected byobjects in the path between the transmitter and receiver. As a result ofone or more reflections, multiple versions of the transmitted signal mayarrive at the receiver at different times. The division of thetransmitted signal into these multiple versions may cause the amplitudeof the transmitted signal to fade at the receiver. If the level offading is great enough, the strength of the signal arriving at thereceiver may be too low for proper receiver processing and the signalmay need to be retransmitted.

[0008] Shadow fading is caused by vehicles moving in and out from behindbuildings, hills, and other obstructions. Shadow fading changes at arate of about once per second.

[0009] Both co-channel interference and multipath fading are frequencydependent. For example, two in-range transmitters may transmit at thesame time without interfering with each other if each transmitter uses adifferent frequency. With respect to multipath fading, some ranges offrequencies are more susceptible to fading than others when transmittedalong the same path. Thus, some wireless communications systemsconstantly “hop” from one available carrier frequency to anotheravailable carrier frequency while transmitting data to avoid theprolonged use of a frequency which might be experiencing highinterference levels. Current frequency hopping systems selectfrequencies at the time a call is initiated. Prior to hopping from onefrequency to another, the transmitting device will usually send amessage to the receiving device so the receiving device will anticipatereceiving data on the new frequency. Depending on the wireless system,the pattern the transmitter follows while hopping among availablefrequency be preplanned, random, pseudo-random, or based upon dynamicfrequency interference level measurements. Further, when a receiverswitches from communicating with one transmitter to communicating withanother transmitter, the frequency hop patterns will likely change.

[0010] Some frequency hopping wireless systems continually measureinterference levels for selected system frequencies during systemoperation. These “dynamic” interference level measurements may be usedto substitute frequencies experiencing high interference levels withfrequencies having lower interference levels. For example, U.S. Pat. No.5,323,447 to Mark E. Gillis et al describes a frequency hopping methodin which a cordless telephone handset measures interference levels amonga first group of frequencies within a frequency hop pattern while usingthe first group of frequencies to communicate with a base unit. Wheninterference is detected on one of the frequencies in the first group,the base station replaces that frequency with a frequency (from a secondgroup of frequencies) having a lower interference level. In anotherexample, U.S. Pat. No. 5,394,433 to David F. Bantz et al, discloses afrequency hopping method in which the entire frequency hop patterncurrently in use by a base station and a mobile station is replaced witha new frequency hop pattern from a predetermined set of patterns upondetecting frequencies with an unacceptable interference level within thecurrent frequency hop pattern.

[0011] Unfortunately, current dynamic frequency hop management methodsmeasure each system frequency sequentially. Due to the rate at whichinterference levels may be sequentially measured for each systemfrequency, these current frequency hop management methods also do notcontemplate measuring all system frequencies at a rate near the rate atwhich the power of a received frequency signal fades while propagatingthrough the transmission medium or the rate at which co-channelinterference charges. The medium through which a frequency signal istransmitted influences the strength of the signal at the receiver. TheRayleigh fading rate is typically used to describe the statistical timevarying nature of frequency signals transmitted; through the air.Although the Rayleigh fading rate covers a range of rates, a fading rateof 100 Hz (which translates to a period of approximately 10 ms) istypically used to describe the rate at which the power of a receivedfrequency signal varies while propagating through the air. Currentfrequency hop management methods are only able to measure interferencelevels for a small portion of the total number of frequencies availableto a typical wireless system within a period during which channel andinterference changes occur. Thus, current frequency management methodsmake replacement decisions by selecting high quality frequencies fromamong fewer than the total number of frequencies available to the systemand make replacement decisions with insufficient knowledge of both thepropagation medium and interference behavior. The ability to measureinterference levels for all available system frequencies at a ratefaster than once per second enables a frequency management method totake full advantage of the potential benefits of frequency hoppingtechniques by selecting best quality frequencies from among all systemfrequencies when attempting to reduce the impact of both propagationmedium and interference influences on the quality of system frequencies.

[0012] Additionally, current frequency hop management methods do noteliminate the possibility that two interfering transmitters within thesame system may respond to measured interference levels bysimultaneously switching to the same high quality frequencies and againinterfering with each other's transmissions.

[0013] In view of the above, it can be appreciated that there is a needfor a method and apparatus that solves the above mentioned problems.

SUMMARY OF THE INVENTION

[0014] The present invention provides a method and apparatus forreducing interference in a frequency hopping wireless communicationsystem. According to an embodiment of the present invention, a widebandtransceiver and an orthogonal frequency division multiplexing (OFDM)technique are used to simultaneously measure an interference level foreach system frequency. After simultaneously measuring interferencelevels for each system frequency using a base station and a terminalstation communicating with the base station, the base station identifiesa frequency hop pattern currently in use for each base station/terminalstation communication link. The measured frequency interference levelsare then used to identify each frequency hop pattern in which at leastone of the current system frequencies should be replaced with a systemfrequency having a lower interference level. The base station thenreplaces no more than a predetermined number of the current systemfrequencies within the identified frequency hop pattern(s) with systemfrequencies having lower interference levels. The above steps areexecuted independently for uplink and downlink frequency hop patterns ateach base station within the wireless system while ensuring that nearbymutually interfering base stations do not replace frequency hop patternfrequencies at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 illustrates a system suitable for practicing an embodimentof the present invention.

[0016]FIG. 2 illustrates an example of a frequency hop pattern composedof six frequency dwells.

[0017]FIG. 3 illustrates a terminal station and a base station suitablefor practicing an embodiment of the present invention.

[0018]FIG. 4 illustrates the terminal station and base station of FIG. 3modified to implement OFDM processing in hardware in accordance with oneembodiment of the present invention.

[0019]FIG. 5 illustrates an example of an OFDM block, in accordance withan embodiment of the present invention.

[0020]FIG. 6 illustrates the terminal station and base station of FIG. 3modified to implement OFDM processing in software in accordance with oneembodiment of the present invention.

[0021]FIG. 7 illustrates a superframe in accordance with an embodimentof the present invention.

[0022]FIG. 8 is a flow chart illustrating an example of the steps forperforming a method in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

[0023] The present invention reduces interference levels within afrequency hopping wireless communication system by dynamically replacingsystem frequencies in use within selected frequency hopping patternswith system frequencies having lower interference levels and byprecluding nearby mutually interfering system components (such as basestations) from simultaneously making frequency replacements using thesame available system frequencies. FIG. 1 is a diagram of a systemsuitable for practicing an embodiment of the present invention. In FIG.1, a geographic area served by a frequency hopping wirelesscommunications system is divided into a plurality of cells 110. In thisembodiment three hexagonal cells 110 are shown. The system can have moreor less than three cells and the cells need not be hexagonal in shape.Each cell 110 includes a base station 102 and one or more terminalstations 104. Terminal stations 104 may be fixed or mobile. Each basestation 102 and terminal station 104 is adapted to transmit and receivevoice and/or data information using radio frequency signals.

[0024] Each base station 102 is adapted to be connected to a mobileswitching center (MSC) 106. MSC 106 is adapted to be connected to afixed network 108. Fixed network 108 may be, for example, a PublicSwitched Telephone Network (PSTN).

[0025] Each base station 102 may select from the entire set of radiofrequencies available to the communications system for use in two-waycommunication with terminal stations 104 located within the geographicarea of the cell 110 in which each base station 102 is located. Two-waycommunication between a base station 102 and a particular terminalstation 104 within the same cell 110 is accomplished by sequentiallymodulating a set of system radio frequencies with voice and/or datainformation. The chronological sequence in which each frequency withinthe set is modulated with voice and/or data information is known as afrequency hop pattern. Each radio frequency within a given frequency hoppattern is modulated with voice and/or data information for a durationof time known as a frequency dwell.

[0026]FIG. 2 illustrates an example of a frequency hop pattern composedof six frequency dwells. In FIG. 2, time is incremented in milliseconds(ms) along the horizontal axis and frequency is incremented in megahertz(MHz) along the vertical axis. The frequency hop pattern of FIG. 2repeats each 60 ms. Each frequency dwell within this frequency hoppattern has a duration of 10 ms. The system frequency in use during eachfrequency dwell of this frequency hop pattern may be determined usingFIG. 2. For example, the 820 MHz system frequency is modulated withvoice and/or data information during the first frequency dwell of thisfrequency hop pattern.

[0027] Each base station 102 in FIG. 1 controls which of the systemradio frequencies are allocated to the frequency hop pattern used tocommunicate with each terminal station 104 within that base station'scell 110. First, the base station 102 selects the frequencies which willbe used to communicate with a particular terminal station 104. The basestation then informs the terminal station 104 of the selectedfrequencies by, for example, transmitting a message to that terminalstation using predetermined designated control frequencies. Similarly,to preclude terminal stations 104 within the same cell fromsimultaneously transmitting voice and/or data information using the samefrequency, each base station 102 controls the sequence of frequencies(i.e., which frequency is used during each frequency dwell) within thefrequency hop patterns used by terminal stations 104 within that basestation's cell 110.

[0028]FIG. 3 illustrates a terminal station and a base station suitablefor practicing an embodiment of the present invention. Terminal station302 is a known device, such as a cellular telephone, modified inaccordance with the present invention. As illustrated in FIG. 3,terminal station 302 comprises a processor 306 adapted to be connectedto a transceiver 308 and a computer readable memory 310. Transceiver 308is adapted to be connected to an antenna 314.

[0029] Computer readable memory 310 stores computer program codesegments which, when executed by processor 306 implement the mainfunctionality for this embodiment of the invention. These computerprogram code segments are included within a quality measurement module312 and a frequency hopping module 328. Although in this embodiment ofthe invention, the computer program code segments are shown in twomodules, it can be appreciated that these modules can be furtherseparated into more modules or combined into one module, and still fallwithin the scope of the invention.

[0030] Base station 304 is a known device modified in accordance with anembodiment of the present invention. As illustrated in FIG. 3, basestation 304 comprises a processor 320 adapted to be connected to acomputer readable memory 322 and a transceiver 318. Transceiver 318 isadapted to be connected to an antenna 316.

[0031] Computer readable memory 322 stores computer program codesegments which, when executed by processor 320 implement the mainfunctionality for this embodiment of the invention. These computerprogram code segments are included within three modules: a qualitymeasurement module 324, a frequency hop pattern adaptation module 326,and a frequency hopping module 330. Although in this embodiment of theinvention, the computer program code segments are shown in threemodules, it can be appreciated that these module can be furtherseparated into more modules or combined into one module, and still fallwithin the scope of the invention.

[0032] By simultaneously (rather than sequentially) measuring aninterference level for each system frequency, the method of the presentinvention obtains frequency interference level measurement values fasterthan current methods. In one embodiment of the present invention,simultaneous system frequency interference level measurements arerapidly obtained using a wideband transceiver and an OFDM technique.FIG. 4 illustrates the terminal station and base station of FIG. 3modified to implement OFDM processing in hardware, in accordance withone embodiment of the present invention. Terminal station 402 includes awideband transceiver 408 and an OFDM block 432 (described in greaterdetail below) in addition to the components described above withreference to terminal station 302 of FIG. 3. Similarly, Base station 404includes a wideband transceiver 418 and an OFDM block 434 (described ingreater detail below) in addition to the components described above withreference to terminal station 304 of FIG. 3.

[0033]FIG. 5 illustrates an example of an OFDM block, in accordance withan embodiment of the present invention. In FIG. 5, an OFDM block 502comprises a serial to parallel conversion device 504 adapted to receivean input signal from a wideband transceiver, such as widebandtransceiver 408 or wideband transceiver 418 of FIG. 4. A fast Fouriertransform (FFT) processing device 506 is adapted to receive “N” inputsignals from serial to parallel conversion device 504. A parallel toserial conversion device 508 is adapted to receiver “N” input signalsfrom FFT processing device 506. Parallel to serial conversion device 508is also adapted to send an output signal to a processor such asprocessor 406 or processor 420 of FIG. 4. OFDM block 502 also comprisesa serial to parallel conversion device 514 adapted to receive an inputsignal from a processor such as processor 406 or processor 420 of FIG.4. An inverse fast fourier transform (IFFT) device 512 is adapted toreceive “N” input signals from serial to parallel conversion device 514.A parallel to serial conversion device 510 is adapted to receive “N”input signals from IFFT processing device 512. Parallel to serialconversion device 510 is also adapted to send an output signal to awideband transceiver such as wideband transceiver 408 or widebandtransceiver 418 of FIG. 4.

[0034] As mentioned previously terminal stations and base stations ofthe present invention are adapted to transmit and receive data. Thus, inone embodiment of the present invention, an OFDM block similar to thatillustrated in FIG. 5 is included within each terminal station and eachbase station of the wireless communication system. To transmit data fromterminal station 402 to base station 404, terminal station 402 providesa high bit rate data stream to an OFDM block within terminal station 402such as OFDM block 502 illustrated in FIG. 5. Serial to parallelconversion device 514 receives the high bit rate data stream and usesthis data stream to generate “N” parallel low bit rate data streams(where “N” is an integer value). The value of “N” is determined by thenumber of frequencies available to the wireless communication system.Serial to parallel conversion device 514 then sends these “N” parallellow bit rate data streams to IFFT processing device 512. IFFT processingdevice 512 uses each of the “N” parallel low bit rate data streams tomodulate “N” different carrier frequencies and then converts each ofthese “N” frequency domain signals to “N” corresponding time domainsignals. IFFT processing device 512 sends these “N” time domain signalsto parallel to serial conversion device 510. Parallel to serialconversion device 510 uses the “N” time domain signals to generate asingle signal comprised of a high bit rate serial stream of data andsends this high bit rate serial data stream signal to a widebandtransceiver or a modulating device where the data stream is used tomodulate a range of carrier frequencies available to the wirelesscommunications system. The modulated signal is then transmitted throughthe air to base station 404.

[0035] Upon receiving the modulated high bit rate serial data streamsignal, base station 404 may use either wideband transceiver 418 oranother demodulating device to demodulate the high bit rate serial datastream signal from the carrier frequency. This high bit rate serial datastream signal is then sent to an OFDM block within base station 404 suchas the OFDM block 502 illustrated in FIG. 5. Serial to parallelconversion device 504 receives the high bit rate serial data stream andconverts this data stream to “N” parallel low bit rate data streamsignals. These “N” parallel low bit rate data stream signals are thensent to FFT processing device 506. FFT processing device 506 uses the“N” parallel low bit rate data stream signals to generate “N” outputsignals, where “N” is the number of frequencies available to thecommunications system. Each system frequency is simultaneouslyrepresented by one output from FFT processing device 506. As describedbelow, signals output from FFT processing device 506 are used tosimultaneously determine the quality of each system frequency. In oneembodiment, the quality of each system frequency is determined bycomparing the relative amplitudes of each signal output from FFTprocessing device 506. If measurements are taken during a period in timewhen no terminal station is transmitting data using a particular systemfrequency, the output signal from FFT processing device 506 whichcorresponds to this particular signal may represent the value ofinterference at that frequency. Thus, the higher the amplitude of thatFFT processing device 506 signal, the higher the interference levelexperienced at that particular frequency. Other methods for representingthe quality of each system frequency such as a ratio of signal to noisemay be obtained in a similar manner.

[0036] The parallel signals output from FFT processing device 506 aresent to parallel to serial conversion device 508. Parallel to serialprocessing device 508 uses these parallel signals to regenerate the highbit rate serial data stream which terminal station 402 sent to basestation 404's OFDM block. Parallel to serial processing device 508 thensends this high bit rate serial data stream to processor 420 for furtherprocessing by base station 404. This further processing may entailsending the data stream to a PSTN through an MSC as illustrated in FIG.1.

[0037] OFDM processing may be implemented in hardware, as describedabove or in software. FIG. 6 illustrates the terminal station and basestation of FIG. 3 modified to implement OFDM processing in software, inaccordance with one embodiment of the present invention. Terminalstation 602 includes a wideband transceiver 608, an analog todigital/digital to analog processing device 636, and an OFDM module 632(described in greater detail below) in addition to the componentsdescribed above with reference to terminal station 302 of FIG. 3.Similarly, base station 604 includes a wideband transceiver 618, ananalog to digital/digital to analog processing device 638, and an OFDMmodule 634 in addition to the components described above with referenceto base station 304 of FIG. 3. Both analog to digital/digital to analogprocessing devices 636 and 638 include components used to convert areceived analog signal to a digital output signal (and vice versa) asappropriate. Analog to digital/digital to analog processing devices 636and 638 transmit signals to and receive signals from OFDM modules 632and 634 respectively via processors 606 and 620 respectively. OFDMmodules 632 and 634 include computer program code segments (as describedabove with reference to FIG. 3) which implement OFDM signal processingin a manner similar to that described above with reference to FIG. 5.

[0038] For example, upon receiving a modulated signal from terminalstation 602, base station 604 first demodulates the signal from thecarrier frequency using wideband transceiver 618 or another demodulationdevice. The demodulated signal is then converted from analog form to acorresponding digital representation using analog to digital/digital toanalog conversion device 638. The digital signal is then processed asdescribed above with reference to FIG. 5 by using processor 620 toexecute the computer program code segments within OFDM module 634.Similar steps are followed in reverse by base station 604 whentransmitting data to terminal station 602.

[0039] An example of the method of the present invention will now bedescribed with reference to FIG. 1, FIG. 4 and FIG. 5. To initialize thesystem, each base station 102 is time synchronized using a system suchas the Global Positioning System. (GPS). Once each base station 102 istime synchronized, a repeating time segment (referred to herein as a“superframe”) is divided into three time segments known as frames andeach base station 102 is assigned to a frame within the superframe. FIG.7 illustrates a superframe in accordance with an embodiment of thepresent invention. The repeating superframe of FIG. 7 is divided intothree frames.

[0040] Once base stations 102 are time synchronized, the following steps(explained below with reference to FIG. 4) are performed at each of thethree base stations 102 of FIG. 1. For the purpose of this example,assume a plurality of terminal stations such as terminal station 402illustrated in FIG. 4 are physically located within the geographic areaof base station 404's cell. First, base station 404 uses qualitymeasurement module 424 to obtain quality measurement values for eachsystem frequency. These quality measurements may be obtained, forexample, through processor 420 from a device such as OFDM block 434.More particularly, these measurements may be obtained from a device suchas FFT processing device 506 (illustrated in FIG. 5) included withinOFDM block 434. Each frequency is represented by an FFT output. In oneembodiment, if a frequency was not being used for transmission during atime period when measurements are obtained from FFT module 506, all theenergy observed at the output corresponding to that frequency representsthe value of interference at that frequency. Thus, the higher theamplitude of the value received from FFT module 506, the higher theinterference level for that frequency. In another embodiment, eachoutput of FFT module 506 represents a ratio of the strength of thefrequency signal to the noise level experienced by that frequencysignal:

[0041] Either on a continuous basis or upon receiving a request frombase station 404 each of the plurality of terminal stations 402 obtainquality measurements for all system frequencies available to thecommunications system. These measurements are accomplished in a mannersimilar to that described above. The plurality of terminal stations 402then send their frequency quality measurements to base station 404.Using both the frequency quality measurements obtained by base station404 and the frequency quality measurements received from the pluralityof terminal stations 402, quality measurement module 424 determines aquality value for each system frequency and assigns a rank number toeach system frequency based upon the determined quality values. The ranknumbers associated with each system frequency increase as the qualityvalue of each frequency decreases. This measurement and ranking isaccomplished independently for uplink and downlink frequency hoppatterns.

[0042] Quality measurement module 424 next retrieves the identity ofeach system frequency used in each frequency dwell of each frequency hoppattern used by each of the plurality of terminal stations 402. Asmentioned previously, base station 404 controls and assigns thefrequencies within the frequency hop patterns implemented by terminalstations 402 within base station 404's cell. Thus, this information maybe retrieved from within computer readable memory 422. Next, qualitymeasurement module 424 assigns the rank number to each frequency dwellwhich corresponds to the system frequency modulated during thatfrequency dwell.

[0043] Quality measurement module 424 then analyzes each frequency hoppattern using the rank information to identify terminal stationfrequency hop patterns in which one or more frequencies should bereplaced with system frequencies having higher quality values (lowerinterference levels). This replacement information is then sent tofrequency hop pattern adaptation module 426. Frequency hop patternadaptation module 426 determines which frequencies should be replacedand informs frequency hopping module 430. Frequency hopping module 430makes the appropriate frequency changes and uses processor 420 totransmit a message to frequency hopping module 428. This messageinstructs frequency hopping module 428 to make the same frequencychanges. Frequency changes within frequency hop patterns also occurindependently for uplink and downlink frequency hop patterns.

[0044] One method for analyzing the frequency hop patterns in use byterminal stations 402 communicating with base station 404 is referred toherein as the “mobile ranking method.” The mobile ranking method entailsfirst assigning a cumulative score to each terminal station. Thecumulative score for a terminal station is obtained by summing the ranknumbers (or the quality measurement values) assigned to the frequencydwells within that terminal station's frequency hop pattern. Eachterminal station is then ranked according to the individually assignedcumulative scores. The terminal station with the worst (highest)cumulative score receives a new frequency hop pattern composed of thebest quality frequencies available for each frequency dwell of thatfrequency hop pattern. The terminal station with the second worst scorereceives a new frequency hop pattern composed of the next best qualityset of frequencies for each individual frequency dwell. This procedureis repeated until the terminal station with the lowest cumulative scorereceives a new frequency hop pattern composed of the remaining bestquality frequencies. In the preferred embodiment, the total number ofavailable system frequencies exceeds the number of frequencies requiredto assign one system frequency to each frequency dwell within eachfrequency hop pattern by such a margin that, during the mobile rankingmethod, the lowest quality frequencies will not be allocated to anyfrequency hop pattern.

[0045] In another embodiment of the present invention, the abovefrequency hop pattern analysis method is modified by comparing thecumulative score assigned to each terminal station to a predeterminedthreshold value. This analysis method is referred to herein as the“threshold based mobile ranking method.” No frequency hop patternreassignments are made for terminal stations having a cumulative scorebelow the threshold value. The frequencies in use by those terminalstations with a cumulative score below the threshold value are notavailable for reassignment to terminal stations having a cumulativescore higher than the threshold value. The terminal stations havingcumulative scores above the threshold value are ranked according totheir cumulative scores. The terminal station with the worst (highest)cumulative score receives a new frequency hop pattern composed of theremaining available best quality frequencies for each frequency dwell ofthat frequency hop pattern. The terminal station with the second worstscore receives a new frequency hop pattern composed of the nextremaining best quality set of frequencies. This procedure is repeateduntil the terminal station (from among the pool of terminal stationshaving a cumulative score above the threshold value) with the lowestcumulative score receives a new frequency hop pattern composed of theremaining best quality frequencies. In the preferred embodiment, thetotal number of available system frequencies exceeds the number offrequencies required to assign one system frequency to each frequencydwell within each frequency hop pattern by such a margin that, duringthe threshold based mobile ranking method, the lowest qualityfrequencies will not be allocated to any frequency hop pattern.

[0046] In another embodiment of the present invention, the frequency hoppatterns in use by terminal stations 402 communicating with base station404 are analyzed by identifying each frequency dwell which includes afrequency having a rank number in the higher end of the range of ranknumbers. In accordance with this method, referred to herein as the“frequency dwell ranking method,” frequencies with higher rank numbersare systematically replaced with frequencies having lower rank numbers.As a higher quality (lower rank number) frequency is used as areplacement, that frequency is removed from the pool of availablereplacement frequencies which may be used in that same frequency dwellby other terminal stations. Removing frequencies from the pool in thismanner ensures no two terminal stations attempt to modulate the samefrequency with voice and/or data information during simultaneouslyoccurring frequency dwells.

[0047] In yet another embodiment of the present invention, an analysismethod referred to herein as the “threshold based frequency dwellranking method” is employed. In this embodiment, the rank of eachfrequency within each frequency dwell is compared to a predeterminedthreshold value. Frequencies having rank numbers below the thresholdvalue remain in use during their current frequency dwell and are removedfrom the pool of available replacement system frequencies. Among thefrequencies having rank numbers above the threshold value, the lowestquality frequencies are systematically replaced with the highest qualityfrequencies. As a higher quality frequency is used as a replacement,that frequency is removed from the pool of available replacement systemfrequencies which may be used in that same frequency dwell by otherterminal stations. Removing frequencies from the pool in this mannerensures no two terminal stations attempt to modulate the same frequencywith voice and/or data information during simultaneously occurringfrequency dwells.

[0048] In yet another embodiment of the present invention, regardless ofthe frequency hop analysis method employed, the number of frequencieswhich may be replaced within any one frequency hop pattern is limited bya predetermined number.

[0049] There are some tradeoffs and advantages associated with choosingfrom among the above four analysis methods. The mobile ranking methodmay be implemented with the least complex algorithm. The threshold basedmobile ranking method requires transmitting the fewest number ofmessages which alert other components of impending frequency changes.The frequency dwell ranking method results in the lowest interferencelevels within the system.

[0050] As mentioned previously, each of the three base stations 102 inFIG. 1 are assigned to a time frame within a superframe. In oneembodiment of the present invention, each base station may only replacefrequencies within frequency hop patterns during the frame to which thatbase station is assigned. This limitation helps reduce the probabilitythat system interference levels will increase due to multiple basestations simultaneously switching to the same high quality systemfrequencies within simultaneously occurring frequency dwells.

[0051]FIG. 8 is a flow chart illustrating an example of the steps forreducing interference within a frequency hopping wireless communicationsystem according to an embodiment of the present invention. The flowchart of FIG. 8 may be implemented, for example, as a computer programor as computer hardware using well-known signal processing techniques.If implemented in software, the computer program instructions may bestored in computer readable memory, such as Read-Only Memory (ROM),Random Access Memory (RAM), magnetic disk (e.g., 3.5″ diskette or harddrive), and optical disk (e.g., CD-ROM). The stored programs may beexecuted, for example, by a general purpose computer which includes aprocessor. More particularly, the steps illustrated in FIG. 8 may beincluded within quality measurement module 424 and frequency hop patternadaptation module 426 illustrated in FIG. 4.

[0052] In step 802, a base station simultaneously determines a qualityvalue for each frequency available to the wireless communication systemand ranks each system frequency as described above. These quality valuesmay be determined using measurements obtained using OFDM methodsimplemented by both a base station and one or more terminal stationsgeographically located within the base station's cell (as describedabove) or may be obtained using OFDM methods implemented by only a basestation. In step 804, the base station identifies each frequency hoppattern in use by each terminal station currently communicating withthis particular base station. In step 806, the base station analyzeseach frequency within each identified frequency hop pattern to ascertainthose frequencies which should be replaced with system frequencieshaving a lower interference value. This step may be executed, forexample, in accordance with one of the four above described analysismethods. In step 808, during the appropriate frame of a superframe, thisparticular base station replaces the ascertained frequencies. The systemexecutes steps 802-808 at each base station within the wirelesscommunications system. This procedure is executed independently foruplink and downlink.

[0053] Although several embodiments are specifically illustrated herein,it will be appreciated that modifications and variations of the presentinvention are covered by the above teachings and within the purview ofthe appended claims without departing from the spirit and intended scopeof the invention. For example, although the method of the presentinvention is described in the context of using OFDM processingtechniques, other techniques may also be used to simultaneously obtaininterference level measurements for each system frequency.

What is claimed is:
 1. A method of determining a measure of aninterference level at each frequency in a plurality of systemfrequencies available to a multichannel wireless communication system,comprising: receiving, at a base station, a plurality of measured valuesrepresentative of an interference level at each frequency in theplurality of system frequencies available to the base station of themultichannel wireless communication system, wherein the interferencelevels were measured simultaneously; measuring, simultaneously, at thebase station, an interference level at each frequency in the pluralityof system frequencies; and calculating a quality value for eachfrequency in the plurality of system frequencies using both theinterference levels received at the base station and the interferencelevels measured at the base station.
 2. The method of claim 1, whereinreceiving occurs one of after a request issued from the base station andcontinuously.
 3. The method of claim 1, wherein measuring isaccomplished using orthogonal frequency division multiplexing (OFDM). 4.A method for reducing interference in a frequency hopping wirelesscommunication system comprising a plurality of base stations eachadapted to communicate with one or more of a plurality of terminalstations by transmitting one or more of a plurality of systemfrequencies through a propagation medium, the method comprising thesteps of: measuring, simultaneously, an interference level for eachsystem frequency; and transmitting values representing said measuredinterference levels through the propagation medium to a base station. 5.The method of claim 4, wherein said measuring step is accomplished usingorthogonal frequency division multiplexing (OFDM).
 6. Acomputer-readable medium whose contents cause a computer system toreduce interference in a wireless communications system comprising aplurality of base stations each adapted to communicate with one or moreof a plurality of terminal stations by transmitting one or more of aplurality of system frequencies through a propagation medium, thecomputer-readable medium performing the steps of: measuring,simultaneously, an interference level for each system frequency; andtransmitting values representing said measured interference levelsthrough the propagation medium to a base station.
 7. Thecomputer-readable medium of claim 6, wherein said measuring step isaccomplished using orthogonal frequency division multiplexing (OFDM).